Quand Marie oublie sa trousse, Paul lui prête un crayon.

Here quand can be understood as when in a general sense, often close to whenever.

So the sentence means something like:

• When/Whenever Marie forgets her pencil case, Paul lends her a pencil.

Because both verbs are in the present tense, it sounds like a habitual or repeated situation, not one single event.

French often uses the present tense to talk about:

• general truths
• habits
• things that happen regularly

So Marie oublie = Marie forgets and Paul prête = Paul lends

This does not necessarily mean it is happening right this second. It can mean this is what usually happens in that situation.

Sa is the possessive adjective meaning his/her/its, and in French it agrees with the noun possessed, not with the owner.

• trousse is a feminine singular noun
• so the possessive form is sa

That is why French says sa trousse.

A useful reminder:

• son
  • masculine singular noun
• sa
  • feminine singular noun
• ses
  • plural noun

So even if the owner were a man, it would still be sa trousse, because trousse is feminine.

In this sentence, trousse means a pencil case or school supply case.

French une trousse can refer to a small case or kit that holds school items like pens and pencils. In a school context, sa trousse most naturally means her pencil case.

Lui here means to her.

In Paul lui prête un crayon:

• Paul = subject
• prête = lends
• un crayon = direct object, the thing being lent
• lui = indirect object, the person receiving it

So literally:

• Paul lends a pencil to her.

Because Marie is the person receiving the pencil, French uses the indirect object pronoun lui.

Because prêter works with an indirect object for the person who receives something.

You lend something to someone:

• direct object = the thing lent
• indirect object = the person receiving it

So in this sentence:

• un crayon = what is lent
• lui = to her

La would be a direct object pronoun, so it would mean her as the direct object, which is not the structure here.

It could repeat à Marie, but French often prefers a pronoun once the person has already been mentioned.

So these are both possible:

• Quand Marie oublie sa trousse, Paul lui prête un crayon.
• Quand Marie oublie sa trousse, Paul prête un crayon à Marie.

The version with lui sounds more natural and less repetitive.

This is a very common point of confusion.

• prêter = to lend
• emprunter = to borrow

So:

• Paul prête un crayon à Marie = Paul lends a pencil to Marie
• Marie emprunte un crayon à Paul = Marie borrows a pencil from Paul

They describe the same situation from opposite points of view.

The first part, Quand Marie oublie sa trousse, is a subordinate clause introduced by quand.

When this kind of clause comes before the main clause, French normally separates it with a comma:

• Quand Marie oublie sa trousse, Paul lui prête un crayon.

If the order were reversed, the comma is often omitted:

• Paul lui prête un crayon quand Marie oublie sa trousse.

Yes. You can also say:

• Paul lui prête un crayon quand Marie oublie sa trousse.

That means the same thing. The original version puts the time/condition idea first, which is very common.

Here it takes the indicative:

• quand Marie oublie...

That is because the sentence refers to a real or usual situation, not something doubtful or emotional. So oublie is simply the present indicative form of oublier.

Un crayon means a pencil, so it introduces a pencil in a general, non-specific way.

That fits the meaning well: Paul lends her a pencil when she needs one.

If you said le crayon, it would sound like a specific pencil already known in the conversation.

Prête is pronounced roughly like pret with an open e sound.

The accent in prête is part of the spelling of this form of prêter. It helps show the vowel sound and distinguishes it from other forms.

Related forms:

• je prête
• tu prêtes
• il/elle prête
• nous prêtons
• vous prêtez
• ils/elles prêtent

So prête here is the third-person singular form: Paul prête.

By itself, it most naturally suggests a habitual or repeated situation:

• Whenever Marie forgets her pencil case, Paul lends her a pencil.

If you wanted to describe one completed event in the past, French would normally use a past tense instead, for example:

• Quand Marie a oublié sa trousse, Paul lui a prêté un crayon.

So the present-tense version usually sounds more general.